Co-pyrolysis of low density polyethylene (LDPE) with three kinds of biomass was performed in a drop down tube reactor. The synergistic effect was investigated by comparing the experimental relative yield and characteristics of oil with the calculated ones based on the results from the pyrolysis of the individual components in the mixture. It is found that the maximum oil relative yield in the case of co-pyrolysis process was obtained at 600 degrees C, which was significantly higher than the optimum temperature of biomass or LDPE pyrolysized alone. Although the inorganic elements in biomass improved the decomposition of LDPE, more organics with larger. molecule weight (>C12) were produced due to the further reaction between the decomposition products from LDPE and biomass. The synergistic effect was positive for the production of aliphatic compounds. However, it could be positive or negative for the production of aromatic compounds, which depended on the type of biomass in the feedstock. The significant removal of aldehydes, acids, ethers, furans, ketones, phenols and sugars from the final co-pyrolysis oil, in which there was a significant increase of alcohols, was achieved. (C) 2016 Elsevier Ltd. All rights reserved.